1. Field of the Invention
The present invention relates to the realization of high performance of a heterojunction bipolar transistor (HBT). More particularly, it relates to the realization of high performance of an HBT having a ballast resistor layer in an emitter layer.
2. Description of the Related Art
FIG. 11 is a sectional view of an HBT based on GaAs/AlGaAs of the conventional structure. An emitter layer consists mainly of an n-AlGaAs layer, a base layer mainly of a p-GaAs layer and a collector mainly of an n-GaAs layer. The emitter layer includes an n-InGaAs contact layer having a narrow band gap and the collector layer includes an n-GaAs contact layer formed therein. In the drawing, the numeral 1 denotes an n.sup.+ -In.sub.0.5 Ga.sub.0.5 As layer, 2 denotes an n.sup.+ -In.sub.x Ga.sub.1-x As (x=0 to 0.5) layer, 3 denotes an n.sup.+ -GaAs layer, 4 denotes an n.sup.- -GaAs ballast resistor layer, 6 denotes an n-Al.sub.y Ga.sub.1-y As layer (y=0 to 0.26), 7 denotes an n-Al.sub.0.26 Ga.sub.0.74 As layer, 8 denotes a non-doped GaAs layer, 9 denotes a p.sup.+ -GaAs base layer, 10 denotes an n.sup.- -GaAs collector layer, 21 denotes an emitter electrode, 22 denotes a base electrode and 23 denotes a collector electrode.
In such a structure as described above, when the device temperature rises due to the HBT operation, the number of electrons flowing through the device, namely the current in the emitter region, increases leading to instability of the device characteristics.
In order to counter this problem, a ballast resistor layer 4 (carrier concentration: 1.times.10.sup.16 cm.sup.-3, film thickness: 2000 .ANG.) which acts as a resistor when the device temperature rises resulting in increased current, is provided between the n.sup.+ -GaAs layer 3 and the n-AlGaAs layer 6 in the emitter region, thereby causing the current to flow through the resistor layer 4 and preventing excessive current from flowing.
Although device characteristics can be prevented from being destabilized, by increasing the device temperature in the HBT which has the ballast resistor layer 4 formed therein, a new problem arises in that the current amplification factor .beta. (rate of change in collector current I.sub.C to a minute change in base current I.sub.B when voltage V.sub.CE between the collector and the emitter is constant) of the HBT decreases compared to the HBT having a conventional structure.
The present inventors found the following fact as a result of an intensive study. That is, in the HBT shown in FIG. 11, since the ballast resistor layer 4 (carrier concentration: 1.times.10.sup.16 cm.sup.-3) is formed on the n-AlGaAs layer 6 (carrier concentration: 5.times.10.sup.17 cm.sup.-3) in the emitter layer, carriers are diffused from the n-AlGaAs layer 6 where the carrier concentration is high to the n.sup.- -GaAs layer of the ballast resistor layer 4, resulting in decreased carrier concentration in the n-AlGaAs layer 6. While such a decrease in the carrier concentration in the n-AlGaAs layer 6 causes the absolute value of the temperature coefficient .phi.(=.delta.V.sub.BE /.delta.T) of the emitter-base voltage V.sub.BE to increase under a condition that the emitter current of the HBT is constant and, as a result, causes the current amplification factor .beta. of the HBT to decrease.